This application relates to the construction of a rotorcraft, such as a helicopter, and more particularly to the concept of using resilient connections between major structural components to alter the natural frequencies of the rotorcraft airframe, thereby reducing excessive loads and vibrations.
During the design and development of rotorcraft airframes, considerable effort is expended for the purpose of ensuring that the frequencies of the major airframe natural modes are sufficiently separated from the frequencies of the driving forces generated by the rotating components. This is necessary to ensure that the dynamic environment of the airframe is acceptable, not only to the crew, but for the operation of on-board equipment, and for adequate fatigue life of the airframe components. For this reason, during the design phase of a new helicopter special attention is paid to frequency placement of the fuselage to keep its natural frequencies away from the rotating system excitation of 1/rev and n/rev. However, since the hardware is not available during this phase, such frequency placements are carried out analytically and the result, considering all the uncertainties regarding the final design, is at best approximate. Ultimately, during shake tests and/or flight tests the actual natural frequencies do not always meet expectations. If these tests prove that the natural frequencies are in fact too close to the excitation frequencies, any alterations to design at this stage are very costly and often require sacrificing some other important features.
In recent years, there has been considerable emphasis on modifying and improving existing rotorcraft platforms, rather than developing completely new designs. Often, the required modifications include mass distribution changes as well as structural changes which impact the airframe stiffness characteristics. These airframe modifications can result in significant and often detrimental changes to the placements of the major airframe modal frequencies. Changes to the rotor speed or changing the number of blades will alter the n/rev excitation frequencies. Sometimes such alteration will cause the natural frequencies of altered aircraft to be placed too close to one of the excitation frequencies of the rotor system.
Prior art solutions for detuning rotorcraft fuselages suffering from the aforementioned problem include fairly draconian measures resulting in significant disadvantages to aircraft performance and/or significant increased cost and complexity. However, it is critically important that the problem be solved by any means available. Thus, in one instance the fuselage was detuned by opening a slot therein to detune the rotorcraft from a resonance condition. The designers were aware of the structural problems such a major structural discontinuity would cause. However, short of a complete redesign of the tailboom, opening the slot was the only feasible solution available.
In another example, an existing helicopter was adapted for operation at higher altitudes by substituting a five bladed rotor for the four bladed rotor originally installed. Unfortunately, the resultant 5/rev frequency fell on the tailboom frequency and was not acceptable. Very expensive self centering servos were required to alleviate the problem.
Therefore, in order to avoid this potential problem and generally accommodate airframe frequency changes for future derivatives of existing rotorcraft, it is desirable that an improved approach be developed for tuning the airframe natural frequencies away from critical driving frequencies. The approach should be readily tailorable to the amount of tuning required, it must be cost-effective as a retrofit, it must not adversely impact other major modal frequencies, and it must not significantly alter the overall operational characteristics of the aircraft. Such an approach should not only be useful for retuning modified aircraft, but also for tuning new designs for which vibration problems are not revealed until the prototype airframe is flight tested.